Silver plating



Patented Oct. 28, 1947 I SILVER PLATING Christian J. Wernlund and James R. Macon, Niagara Falls, N. Y., assignors to E. I. du Pont de Nemours & Company, Wilmington, Del., a

corporation of Delaware No Drawing. Application January 11, 1944, Serial No. 517,841

Claims. 1

This invention relates to silver plating and more particularly, to the electrodeposition of bright silver deposits from silver cyanide plating baths.

Without the use of brightening agents, silver cyanide plating baths produce dull or matte type silver deposits. Heretofore carbon disulfide has been used to some extent as a brightening agent in silver plating baths. However, the brightening effect of this addition agent is effective only over a, relatively narrow range of current densities and it has not been entirely suitable for the electrodeposition at relatively high current densities for a variety of purposes.

An object of this bath is to provide a, novel process for the electrodeposition of bright silver electrodeposits. A further object is to provide a brightening agent for silver cyanide plating baths which is effective over a relatively wide current density range and at relatively high .current densities. Further objects will be apparent from the following description of the invention.

We have discovered that excellent bright deposits of silver can be obtained over a wide range of current densities by electrolysis of silver cyanide solutions to which have been added small amounts of certain organic sulfur compounds, to wit: Z-mercaptobenzothiazole, 2-mercapt0thiazole, 2-mercaptothiazoline, thioacetanilide and trimercaptocyanuric acid. Our novel addition agents may be utilized inany conventional silver cyanide plating bath, that is, a bath containing a double cyanide of silver and an alkali metal cyanide, with or without the further addition of alkali metal hydroxide, carbonate or other additives common in silver cyanide plating solutions. The following is an example of a silver cyanide plating bath to which our invention can be applied, it being understood that this formula is given merely by way of example and thatthe invention is in nowise restricted thereto:

Ounces per gallon Silver plating baths made with potassium cyanide are generally preferred in practicing our invention, but our novel brightening agents are likewise efiective in baths made with sodium cyanide or with the cyanide of the other alkali metals, cesium, rubidium and lithium. We may also use electrolytes containing ions of two or more of the alkali metals, in any desired proportion. We may use a high concentration of free cyanide, e. g., to 14 ounces per gallon, or substantially no free cyanide. In general, an increase in free cyanide concentration results in a corresponding widening of the current density bright range. Usually we prefer to use about 6 2. to 12 ounces per gallon of free cyanide. Various other practices common in electroplating silver 7 from cyanide baths are generally applicable to our invention.

In practicing our invention we may add from 0.01 to about 1 ounce per gallon of the addition agent. Preferably, the addition agent is first dissolved in a small amount of alkali metal hydroxideor in a small amount of the electroplating solution and the appropriate amount of this soltion is added to the plating bath. The bath is then operated in the conventional manner to electrodeposit silver.

We have found that by means of our addition agents good, bright deposits of silver may be obtained with still electrodes at current densities as high as 30 amperes per square foot at both the anode and cathode, and with moving electrodes or with bath agitation to cause the electrolyte to flow past the electrodes, at current densities as high as 100 amperes per square foot at the anode and up to amperes per square foot at the cathode. With a bath such as that set forth above, operated at a bath temperature of 21 to 43 C., we are able to electrodeposit 0.001 inch of silver at a current density of 37 amperes per square foot at the cathode in a period of about 10 minutes. The electrodeposit is hard, dense and mirror bright, Th brightness of the electrodeposit and the high speed with which it is deposited make the process especially attractive for many present day industrial uses,

The following examples further illustrate our invention:

Ounces per gallon AgCN 10 KCN 1'7 KOH 0.25

Bright electrodeposits were obtained at the cathode current densities indicated below:

Z-Mercaptobenzothiazole Concentration Current Density Bright Range 02. per gal. Amps per sq.ft.

4 to '0. 067 5 to over 0. 134 6 to over 70 AgCN Example 2 The procedure of Example 1 was carried out using the following solution:

Ounces per gallon Silver was electroplated from the following solution maintained at a temperature of 32 C., to which was added 2-mercaptothiazo1e:

Ounces per gallon AgCN 10 KCN 10 Bright electrodeposits were obtained at the cathode current densities indicated below:

2Memapto' Current Densit thiazole y Concentration Brlght Range 02.1781 gal. Amps. per saft.

. 0 0. 027 to 0. 040 5 to 17 0.067 5 to 17 Example 4 Silver was electroplated from the following solution, maintained at a temperature of 33 0., to which was added 2-mercaptothiazo1ine:

Ounces per gallon AgCN 10 KCN 1o KOH 4 Bright electrodeposits were obtained at the cathode current densities indicated below:

2-Mcrcapt0- thiazoline Concentration Current Density Bright Range 02. per gal.

Amps. per sq.ft. 25 to 30 0. 027

30 to 55 0. 054 30 to 60 Example 5 Silver was electroplated from the following solution, to which was added thioacetanilide:

Ounces per gallon 10 KCN 10 Bright electrodeposits were obtained at the cathode current densities indicated below:

Thioacetanilide Current Destiny Concentration Bright Range 02. per gal. Amps. per daft.

0.027 50 to over 70 0. 054 11 to 45 Example 6 Silver was electroplated from the solution of Example 5, except that trimercaptocyanuric acid was added as brightener. At a bath temperature of 32 C. and at the cathode current densities indicated below, bright electrodeposits were obtained:

We claim:

1. A process for producing bright silver electrodeposits which comprises electrolyzing a silver-alkali metal cyanide solution containing an organic sulfur compound selected from the group consisting of Z-mercaptobenzothiazole, 2-mercaptothiazole, 2-mercaptothiazoline, thioacetanilide and trimercaptocyanuric acid in an amount sufficient to brighten the deposit within the range of 0.01 to 1 ounce per gallon.

2. A process for producing bright silver electrodeposits which comprises electrolyzing a silver-alkali metal cyanide solution containing 2- mercaptobenzothiazole in an amount sufficient to brighten the deposit within the range of 0.01 to 1 ounce per gallon.

3. A process for producing bright silver electrodeposits which comprises electrolyzing a, silyer-alkali metal cyanide solution containing thioacetanilide in an amount sufiicient to brighten the deposit Within the range of 0.01 to 1 ounce per gallon.

4. A process for producing bright silver electrodeposits which comprises electrolyzing a silver-alkali metal cyanide solution containing trimercaptocyanuric acid in an amount suflicient to brighten the deposit within the range of 0.01 to 1 ounce per gallon.

5. A process for producing bright silver electrodeposits which comprises electrolyzing a silver-potassium cyanide solution containing 2- mercaptobenzothiazole in an amount sufiicient to brighten the deposit within the range of 0.01 to 0.5 ounce per gallon.

CHRISTIAN J. WERNLUND. JAMES R. MACON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,857,507 Heckman et al. May 10, 1932 2,110,792 Egeberg et a1 Mar. 8, 1938 OTHER REFERENCES Transactions of the ElectrochemicalSociety, vol. 74, pages 211-227 (1938). 

